tests/docker: peg netmap code to a specific version

Tracking head is always going to be at the whims of the upstream.
Let's use a defined release so things don't magically change under us.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/tests/docker/dockerfiles/debian-amd64.docker b/tests/docker/dockerfiles/debian-amd64.docker
index 954fcf9..d770a11 100644
--- a/tests/docker/dockerfiles/debian-amd64.docker
+++ b/tests/docker/dockerfiles/debian-amd64.docker
@@ -33,6 +33,7 @@
     apt-get install -y --no-install-recommends \
         linux-headers-amd64
 RUN git clone https://github.com/luigirizzo/netmap.git /usr/src/netmap
+RUN cd /usr/src/netmap && git checkout v11.3
 RUN cd /usr/src/netmap/LINUX && ./configure --no-drivers --no-apps --kernel-dir=$(ls -d /usr/src/linux-headers-*-amd64) && make install
 ENV QEMU_CONFIGURE_OPTS --enable-netmap